1. Field of the Invention
The present invention relates to a jet-propulsion personal watercraft (PWC).
2. Description of the Related Art
In recent years, jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is equipped with an engine within a space surrounded by a hull and a deck. The personal watercraft is configured to have a water jet pump that pressurizes and accelerates water sucked from a water intake generally provided on a hull bottom surface and ejects it rearward from an outlet port of the water jet pump. As the resulting reaction, the personal watercraft is propelled forward.
In the jet-propulsion personal watercraft, a steering nozzle provided behind the outlet port of the water jet pump is swung either to the right or to the left by operating a bar-type steering handle to the right or to the left, to change the ejection direction of the water to the right or to the left, thereby turning the watercraft to right or to the left.
In the personal watercraft, various devices are made to gain a high power using the same engine body. As a prior art, Japanese Laid-Open Patent Application Publication No. 2001-146197 (pages 1 to 8, FIGS. 5 and 6) discloses a personal watercraft in which a turbocharger is mounted to efficiently gain a high power engine. The turbocharger is a supercharger, and is configured to rotate a built-in turbine using flow of an exhaust gas of the engine for air taken in from outside to be pressurized and to supply the pressurized air to the engine. With this configuration, a large amount of air is supplied to the engine, and thereby air-intake efficiency can be increased.
While traveling at high speeds in choppy water, the personal watercraft tends to frequently jump out of the water and land in the water. When the watercraft is jumping, the water jet pump rotates without water in the pump, thereby causing a load applied to the engine to rapidly decrease. Under this condition, since the engine runs at a high speed, the turbine of the turbo charger rotates at a high speed, thereby causing a supercharging pressure to rapidly increase.
The turbocharger is typically provided with a relief valve for air release to inhibit the supercharging pressure from excessively increasing. When the watercraft is jumping out of the water and the supercharging pressure becomes relatively high, the relief valve reduces the pressure of the air, so that the supercharging pressure is reduced when the watercraft lands in the water. Therefore, in the case where the watercraft frequently jumps out of the water and lands in the water, a rider typically opens a throttle valve after landing to allow the engine speed to increase, for the purpose of increasing the reduced supercharging pressure.
However, since the turbocharger pressurizes the taken-in air utilizing a turbine rotated by the flow of exhaust gas, an effect peculiar to turbochargers, referred to as “turbo lag,” takes place, which causes delay in the response to the rider's throttle operation. Therefore, when the watercraft frequently jumps and lands in choppy water, the turbo lag takes place each time the watercraft lands and the rider opens the throttle valve. Consequently, when the personal watercraft in which the turbocharger is mounted travels in choppy water, it takes some time due to the turbo lag to increase a propulsion force after the watercraft lands.